This class of device is described, for example, in a text entitled "Laser Applications" edited by Monte Ross, Academic Press, Inc., New York, N.Y., 1971 in which pages 134 to 200 relating to "The Laser Gyro" are particularly to be noted.
It is well known that two counterrotating laser beams may be established in a ring type laser. When the ring laser is rotated about a particular axis passing centrally through the ring laser, the frequency of the oscillations are shifted, with the frequency of the beam travelling in the direction of rotation exhibiting a decrease, and the frequency of the beam travelling in the direction opposite to the rotation exhibiting an increase. The amount of rotation may be determined by detecting the beat frequencies between the counterrotating beams. Using a set of three laser gyros, having mutually perpendicular sensing axes, this phenomenon may be employed in inertial guidance systems to determine the rotation and the resultant orientation of an airplane or the like.
In ring lasers of the type mentioned above, the reflecting surfaces are of dielectric materials and are built up of from 15 to 30 layers, in the usual cases. They are intended to provide proper reflection characteristics for both the P mode of polarization (P having the electric vector parallel to the plane of incidence) and for the S polarized components (where the electric vector is orthogonal to the P vector). Unfortunately it is almost impossible to produce reflecting surfaces or mirrors having little or no phase anisotropy at the desired angle of incidence. Further, temperature and other changes affect the anisotropy. More specifically, manufacturers of such dielectric mirrors are unable consistently to produce dielectric mirrors having phase and attenuation isotropic characteristics for P and S polarization at the desired angles of incidence, such as 30.degree. for a three mirror ring laser and 45.degree. for a four mirror ring laser.
When ring lasers are constructed using these imperfect or anisotropic mirrors, there is strong coupling between the modes, and it is difficult to achieve multioscillator operation.